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<tr class="t-nv"><td colspan="5"> <a href="const_cast.html" title="cpp/language/const cast"><tt>const_cast</tt></a> - <a href="reinterpret_cast.html" title="cpp/language/reinterpret cast"><tt>reinterpret_cast</tt></a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Memory allocation</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="new.html" title="cpp/language/new"> <code>new</code> expression</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="delete.html" title="cpp/language/delete"> <code>delete</code> expression</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Classes</a></td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class.html" title="cpp/language/class"> Class declaration</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initializer_list.html" title="cpp/language/constructor"> Constructors</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this"> <code>this</code> pointer</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access"> Access specifiers</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"> <code>friend</code> specifier</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Class-specific function properties</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual"> Virtual function</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"> <code>override</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"> <code>final</code> specifier</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><tt>explicit</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static"><tt>static</tt></a> </td></tr>
</table></div></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions</td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor"> Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor"> Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor"> Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/as operator" class="mw-redirect"> Copy assignment</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move operator" class="mw-redirect"> Move assignment</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor"> Destructor</a></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"><a href="templates.html" title="cpp/language/templates"> Templates</a> </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="class_template.html" title="cpp/language/class template"> Class template </a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="function_template.html" title="cpp/language/function template"> Function template </a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="template_specialization.html" title="cpp/language/template specialization"> Template specialization</a></td></tr>   
<tr class="t-nv"><td colspan="5"> <a href="parameter_pack.html" title="cpp/language/parameter pack"> Parameter packs</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div></td></tr>
<tr class="t-nv-h1"><td colspan="5"> Miscellaneous </td></tr>
<tr class="t-nv-col-table"><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="asm.html" title="cpp/language/asm"> Inline assembly</a></td></tr>
</table></div></td><td><div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv"><td colspan="5"> <a href="history.html" title="cpp/language/history"> History of C++</a></td></tr>
</table></div></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div><div class="t-navbar-head"><a href="classes.html" title="cpp/language/classes"> Classes</a><div class="t-navbar-menu"><div><div style="display:inline-block">
<div><table class="t-nv-begin" cellpadding="0" style="">
<tr class="t-nv-h2"><td colspan="5"> General </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="classes.html" title="cpp/language/classes"> Overview</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="class.html" title="cpp/language/class"> <code>class</code>/<code>struct</code> types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="union.html" title="cpp/language/union"> <code>union</code> types</a> </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="injected-class-name.html" title="cpp/language/injected-class-name"> Injected-class-name</a> </td></tr>
<tr class="t-nv-h2"><td colspan="5"> Members </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="data_members.html" title="cpp/language/data members">Data members</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="static.html" title="cpp/language/static">Static members</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="this.html" title="cpp/language/this">The <code>this</code> pointer</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="nested_classes.html" title="cpp/language/nested classes" class="mw-redirect">Nested classes</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="member_template.html" title="cpp/language/member template">Member templates</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="bitfield.html" title="cpp/language/bit field">Bit fields</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="using_declaration.html" title="cpp/language/using declaration"><code>using</code>-declarations</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="member_functions.html" title="cpp/language/member functions">Member functions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="access.html" title="cpp/language/access">Member access specifiers</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="initializer_list.html" title="cpp/language/constructor">Constructors and member initializer lists</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="data_members.html#Member_initialization" title="cpp/language/data members">Default member initializer</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="friend.html" title="cpp/language/friend"><code>friend</code> specifier</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="explicit.html" title="cpp/language/explicit"><code>explicit</code> specifier</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="converting_constructor.html" title="cpp/language/converting constructor">Converting constructor</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Special member functions </td></tr>
<tr class="t-nv"><td colspan="5"> <a href="default_constructor.html" title="cpp/language/default constructor">Default constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="copy_constructor.html" title="cpp/language/copy constructor">Copy constructor</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_constructor.html" title="cpp/language/move constructor">Move constructor</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="as_operator.html" title="cpp/language/copy assignment">Copy assignment operator</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="move_operator.html" title="cpp/language/move assignment">Move assignment operator</a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="destructor.html" title="cpp/language/destructor">Destructor</a></td></tr>
<tr class="t-nv-h2"><td colspan="5"> Inheritance </td></tr>
<tr class="t-nv"><td colspan="5"> <strong class="selflink">Base and derived classes</strong></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="ebo.html" title="cpp/language/ebo">Empty base optimization (EBO)</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="virtual.html" title="cpp/language/virtual">Virtual member functions</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="abstract_class.html" title="cpp/language/abstract class">Pure virtual functions and abstract classes</a></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="override.html" title="cpp/language/override"><tt>override</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
<tr class="t-nv"><td colspan="5"> <a href="final.html" title="cpp/language/final"><tt>final</tt></a> <span class="t-mark-rev t-since-cxx11">(C++11)</span></td></tr>
</table></div>
</div><div></div></div></div></div><div class="t-navbar-sep"> </div></div>
<p>Any class type (whether declared with <span class="t-spar">class-key</span> <code>class</code> or <code>struct</code>) may be declared as <i>derived</i> from one or more <i>base classes</i> which, in turn, may be derived from their own base classes, forming an inheritance hierarchy.
</p><p>The list of base classes is provided in the <span class="t-spar">base-clause</span> of the <a href="class.html" title="cpp/language/class">class declaration syntax</a>. The <span class="t-spar">base-clause</span> consists of the character <code><b>:</b></code> followed by a comma-separated list of one or more <span class="t-spar">base-specifier</span>s.
</p>
<table class="t-sdsc-begin">

<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">class-or-decltype</span>
</td>
<td> (1)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <code><b>virtual</b></code> <span class="t-spar">class-or-decltype</span>
</td>
<td> (2)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">access-specifier</span> <span class="t-spar">class-or-decltype</span>
</td>
<td> (3)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <code><b>virtual</b></code> <span class="t-spar">access-specifier</span> <span class="t-spar">class-or-decltype</span>
</td>
<td> (4)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr>
<tr class="t-sdsc">
<td> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">access-specifier</span> <code><b>virtual</b></code> <span class="t-spar">class-or-decltype</span>
</td>
<td> (5)
</td>
<td class="t-sdsc-nopad">
</td></tr>



<tr>
<td colspan="10" class="t-sdsc-sep">
</td></tr></table>
<div class="t-li1"><span class="t-li">1)</span> Specifies a non-virtual inheritance with default member accessibility.</div>
<div class="t-li1"><span class="t-li">2)</span> Specifies a virtual inheritance with default member accessibility.</div>
<div class="t-li1"><span class="t-li">3)</span> Specifies a non-virtual inheritance with given member accessibility.</div>
<div class="t-li1"><span class="t-li">4)</span> Specifies a virtual inheritance with given member accessibility.</div>
<div class="t-li1"><span class="t-li">5)</span> Same as 4), <code><b>virtual</b></code> and <span class="t-spar">access-specifier</span> can appear in any order.</div>
<table class="t-par-begin">


<tr class="t-par">
<td> <span class="t-spar">attr</span>
</td>
<td> -
</td>
<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span> sequence of any number of <a href="attributes.html" title="cpp/language/attributes">attributes</a>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">access-specifier</span>
</td>
<td> -
</td>
<td> one of <code>private</code>, <code>public</code>, or <code>protected</code>
</td></tr>
<tr class="t-par">
<td> <span class="t-spar">class-or-decltype</span>
</td>
<td> -
</td>
<td> one of
<ul><li> <span class="t-spar">nested-name-specifier</span><span class="t-mark">(optional)</span> <span class="t-spar">type-name</span>
</li><li> <span class="t-spar">nested-name-specifier</span> <code><b>template</b></code> <span class="t-spar">simple-template-id</span>
</li></ul>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<ul><li> <a href="decltype.html" title="cpp/language/decltype"><span class="t-spar">decltype-specifier</span></a>
</li></ul>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
</td></tr></table>
<p>An <a href="elaborated_type_specifier.html" title="cpp/language/elaborated type specifier">elaborated type specifier</a> cannot directly appear as <span class="t-spar">class-or-decltype</span> due to syntax limitations.
</p>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11"><td>
<p><span class="t-spar">base-specifier</span>s in a <span class="t-spar">base-clause</span> may be <a href="parameter_pack.html" title="cpp/language/parameter pack">pack expansions</a>.
</p><p>A class or struct declared <a href="final.html" title="cpp/language/final"><code>final</code></a> cannot be denoted by <span class="t-spar">class-or-decltype</span>.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td></tr>
</table>
<p>If <span class="t-spar">access-specifier</span> is omitted, it defaults to <code>public</code> for classes declared with <span class="t-spar">class-key</span> <code>struct</code> and to <code>private</code> for classes declared with <span class="t-spar">class-key</span> <code>class</code>.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> Base
<span class="br0">{</span>
    <span class="kw4">int</span> a, b, c<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// every object of type Derived includes Base as a subobject</span>
<span class="kw1">struct</span> Derived <span class="sy4">:</span> Base
<span class="br0">{</span>
    <span class="kw4">int</span> b<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// every object of type Derived2 includes Derived and Base as subobjects</span>
<span class="kw1">struct</span> Derived2 <span class="sy4">:</span> Derived
<span class="br0">{</span>
    <span class="kw4">int</span> c<span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>Classes denoted by <span class="t-spar">class-or-decltype</span>'s listed in the <span class="t-spar">base-clause</span> are direct base classes. Their bases are indirect base classes. The same class cannot be specified as a direct base class more than once, but the same class can be both direct and indirect base class.
</p><p>Each direct and indirect base class is present, as <i>base class subobject</i>, within the object representation of the derived class at ABI-dependent offset. Empty base classes usually do not increase the size of the derived object due to <a href="ebo.html" title="cpp/language/ebo">empty base optimization</a>. The constructors of base class subobjects are called by the constructor of the derived class: arguments may be provided to those constructors in the <a href="initializer_list.html" title="cpp/language/initializer list" class="mw-redirect">member initializer list</a>. 
</p>
<h3><span class="mw-headline" id="Virtual_base_classes">Virtual base classes</span></h3>
<p>For each distinct base class that is specified <code>virtual</code>, the most derived object contains only one base class subobject of that type, even if the class appears many times in the inheritance hierarchy (as long as it is inherited <code>virtual</code> every time).
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> B <span class="br0">{</span> <span class="kw4">int</span> n<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> X <span class="sy4">:</span> <span class="kw1">public</span> <span class="kw1">virtual</span> B <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> Y <span class="sy4">:</span> <span class="kw1">virtual</span> <span class="kw1">public</span> B <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">class</span> Z <span class="sy4">:</span> <span class="kw1">public</span> B <span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// every object of type AA has one X, one Y, one Z, and two B's:</span>
<span class="co1">// one that is the base of Z and one that is shared by X and Y</span>
<span class="kw1">struct</span> AA <span class="sy4">:</span> X, Y, Z
<span class="br0">{</span>
    AA<span class="br0">(</span><span class="br0">)</span>
    <span class="br0">{</span>
        X<span class="sy4">::</span><span class="me2">n</span> <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// modifies the virtual B subobject's member</span>
        Y<span class="sy4">::</span><span class="me2">n</span> <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span> <span class="co1">// modifies the same virtual B subobject's member</span>
        Z<span class="sy4">::</span><span class="me2">n</span> <span class="sy1">=</span> <span class="nu0">3</span><span class="sy4">;</span> <span class="co1">// modifies the non-virtual B subobject's member</span>
 
        <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> X<span class="sy4">::</span><span class="me2">n</span> <span class="sy1">&lt;&lt;</span> Y<span class="sy4">::</span><span class="me2">n</span> <span class="sy1">&lt;&lt;</span> Z<span class="sy4">::</span><span class="me2">n</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// prints 223</span>
    <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span></pre></div></div>
<p>An example of an inheritance hierarchy with virtual base classes is the iostreams hierarchy of the standard library: <span class="t-lc"><a href="../io/basic_istream.html" title="cpp/io/basic istream">std::istream</a></span> and <span class="t-lc"><a href="../io/basic_ostream.html" title="cpp/io/basic ostream">std::ostream</a></span> are derived from <span class="t-lc"><a href="../io/basic_ios.html" title="cpp/io/basic ios">std::ios</a></span> using virtual inheritance. <span class="t-lc"><a href="../io/basic_iostream.html" title="cpp/io/basic iostream">std::iostream</a></span> is derived from both <span class="t-lc"><a href="../io/basic_istream.html" title="cpp/io/basic istream">std::istream</a></span> and <span class="t-lc"><a href="../io/basic_ostream.html" title="cpp/io/basic ostream">std::ostream</a></span>, so every instance of <span class="t-lc"><a href="../io/basic_iostream.html" title="cpp/io/basic iostream">std::iostream</a></span> contains a <span class="t-lc"><a href="../io/basic_ostream.html" title="cpp/io/basic ostream">std::ostream</a></span> subobject, a <span class="t-lc"><a href="../io/basic_istream.html" title="cpp/io/basic istream">std::istream</a></span> subobject, and just one <span class="t-lc"><a href="../io/basic_ios.html" title="cpp/io/basic ios">std::ios</a></span> subobject (and, consequently, one <span class="t-lc"><a href="../io/ios_base.html" title="cpp/io/ios base">std::ios_base</a></span>).
</p><p>All virtual base subobjects are initialized before any non-virtual base subobject, so only the most derived class calls the constructors of the virtual bases in its <a href="initializer_list.html" title="cpp/language/initializer list" class="mw-redirect">member initializer list</a>:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">struct</span> B
<span class="br0">{</span>
    <span class="kw4">int</span> n<span class="sy4">;</span>
 
    B<span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="sy4">:</span> n<span class="br0">(</span>x<span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw1">struct</span> X <span class="sy4">:</span> <span class="kw1">virtual</span> B <span class="br0">{</span> X<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> B<span class="br0">(</span><span class="nu0">1</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> Y <span class="sy4">:</span> <span class="kw1">virtual</span> B <span class="br0">{</span> Y<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> B<span class="br0">(</span><span class="nu0">2</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
<span class="kw1">struct</span> AA <span class="sy4">:</span> X, Y     <span class="br0">{</span> AA<span class="br0">(</span><span class="br0">)</span> <span class="sy4">:</span> B<span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span>, X<span class="br0">(</span><span class="br0">)</span>, Y<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// the default constructor of AA calls the default constructors of X and Y</span>
<span class="co1">// but those constructors do not call the constructor of B because B is a virtual base</span>
AA a<span class="sy4">;</span> <span class="co1">// a.n == 3</span>
 
<span class="co1">// the default constructor of X calls the constructor of B</span>
X x<span class="sy4">;</span>  <span class="co1">// x.n == 1</span></pre></div></div>
<p>There are <a href="unqualified_lookup.html#Member_function_definition" title="cpp/language/unqualified lookup">special rules</a> for unqualified name lookup for class members when virtual inheritance is involved (sometimes referred to as the rules of dominance).
</p>
<h3><span class="mw-headline" id="Public_inheritance">Public inheritance</span></h3>
<p>When a class uses <code>public</code> <a href="access.html" title="cpp/language/access">member access specifier</a> to derive from a base, all public members of the base class are accessible as public members of the derived class and all protected members of the base class are accessible as protected members of the derived class (private members of the base are never accessible unless friended)
</p><p>Public inheritance models the subtyping relationship of object-oriented programming: the derived class object IS-A base class object. References and pointers to a derived object are expected to be usable by any code that expects references or pointers to any of its public bases (see <a href="https://en.wikipedia.org/wiki/Liskov_substitution_principle" class="extiw" title="enwiki:Liskov substitution principle">LSP</a>) or, in <a href="https://en.wikipedia.org/wiki/Design_by_contract" class="extiw" title="enwiki:Design by contract">DbC</a> terms, a derived class should maintain class invariants of its public bases, should not strengthen any precondition or weaken any postcondition of a member function it <a href="virtual.html" title="cpp/language/virtual">overrides</a>.
</p>
<div class="t-example"><div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;vector&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="co2">#include &lt;iostream&gt;</span>
 
<span class="kw1">struct</span> MenuOption <span class="br0">{</span> <a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a> title<span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// Menu is a vector of MenuOption: options can be inserted, removed, reordered...</span>
<span class="co1">// and has a title.</span>
<span class="kw1">class</span> Menu <span class="sy4">:</span> <span class="kw1">public</span> <a href="../container/vector.html"><span class="kw1272">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span>MenuOption<span class="sy1">&gt;</span>
<span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
    <a href="../string/basic_string.html"><span class="kw1233">std::<span class="me2">string</span></span></a> title<span class="sy4">;</span>
 
    <span class="kw4">void</span> print<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span>
    <span class="br0">{</span>
        <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> title <span class="sy1">&lt;&lt;</span> <span class="st0">":<span class="es1">\n</span>"</span><span class="sy4">;</span>
        <span class="kw1">for</span> <span class="br0">(</span><a href="../types/size_t.html"><span class="kw106">std::<span class="me2">size_t</span></span></a> i <span class="sy1">=</span> <span class="nu0">0</span>, s <span class="sy1">=</span> size<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> i <span class="sy1">&lt;</span> s<span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span>
            <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"  "</span> <span class="sy1">&lt;&lt;</span> <span class="br0">(</span>i<span class="sy2">+</span><span class="nu0">1</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">". "</span> <span class="sy1">&lt;&lt;</span> at<span class="br0">(</span>i<span class="br0">)</span>.<span class="me1">title</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
    <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// Note: Menu::title is not problematic because its role is independent of the base class.</span>
 
<span class="kw2">enum</span> <span class="kw1">class</span> Color <span class="br0">{</span> WHITE, RED, BLUE, GREEN <span class="br0">}</span><span class="sy4">;</span>
 
<span class="kw4">void</span> apply_terminal_color<span class="br0">(</span>Color<span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/* OS-specific */</span> <span class="br0">}</span>
 
<span class="co1">// THIS IS BAD!</span>
<span class="co1">// ColorMenu is a Menu where every option has a custom color.</span>
<span class="kw1">class</span> ColorMenu <span class="sy4">:</span> <span class="kw1">public</span> Menu
<span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
    <a href="../container/vector.html"><span class="kw1272">std::<span class="me2">vector</span></span></a><span class="sy1">&lt;</span>Color<span class="sy1">&gt;</span> colors<span class="sy4">;</span>
 
    <span class="kw4">void</span> print<span class="br0">(</span><span class="br0">)</span> <span class="kw4">const</span>
    <span class="br0">{</span>
        <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> title <span class="sy1">&lt;&lt;</span> <span class="st0">":<span class="es1">\n</span>"</span><span class="sy4">;</span>
 
        <span class="kw1">for</span> <span class="br0">(</span><a href="../types/size_t.html"><span class="kw106">std::<span class="me2">size_t</span></span></a> i <span class="sy1">=</span> <span class="nu0">0</span>, s <span class="sy1">=</span> size<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> i <span class="sy1">&lt;</span> s<span class="sy4">;</span> <span class="sy2">++</span>i<span class="br0">)</span>
        <span class="br0">{</span>
            <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"  "</span> <span class="sy1">&lt;&lt;</span> <span class="br0">(</span>i<span class="sy2">+</span><span class="nu0">1</span><span class="br0">)</span> <span class="sy1">&lt;&lt;</span> <span class="st0">". "</span><span class="sy4">;</span>
            apply_terminal_color<span class="br0">(</span>colors<span class="br0">[</span>i<span class="br0">]</span><span class="br0">)</span><span class="sy4">;</span>
            <a href="../io/cout.html"><span class="kw1762">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> at<span class="br0">(</span>i<span class="br0">)</span>.<span class="me1">title</span> <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
            apply_terminal_color<span class="br0">(</span>Color<span class="sy4">::</span><span class="me2">WHITE</span><span class="br0">)</span><span class="sy4">;</span>
        <span class="br0">}</span>
    <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
<span class="co1">// ColorMenu needs the following invariants that cannot be satisfied</span>
<span class="co1">// by publicly inheriting from Menu, for example:</span>
<span class="co1">// - ColorMenu::colors and Menu must have the same number of elements</span>
<span class="co1">// - To make sense, calling erase() should remove also elements from colors,</span>
<span class="co1">//   in order to let options keep their colors</span>
<span class="co1">// Basically every non-const call to a std::vector method will break the invariant</span>
<span class="co1">// of the ColorMenu and will need fixing from the user by correctly managing colors.</span>
 
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    ColorMenu color_menu<span class="sy4">;</span>
 
    <span class="co1">// The big problem of this class is that we must keep ColorMenu::Color</span>
    <span class="co1">// in sync with Menu.</span>
    color_menu.<span class="me1">push_back</span><span class="br0">(</span>MenuOption<span class="br0">{</span><span class="st0">"Some choice"</span><span class="br0">}</span><span class="br0">)</span><span class="sy4">;</span>
 
    <span class="co1">// color_menu.print(); // ERROR! colors[i] in print() is out of range</span>
 
    color_menu.<span class="me1">colors</span>.<span class="me1">push_back</span><span class="br0">(</span>Color<span class="sy4">::</span><span class="me2">RED</span><span class="br0">)</span><span class="sy4">;</span>
 
    color_menu.<span class="me1">print</span><span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// OK: colors and Menu has the same number of elements</span>
<span class="br0">}</span></pre></div></div>
</div>
<h3><span class="mw-headline" id="Protected_inheritance">Protected inheritance</span></h3>
<p>When a class uses <code>protected</code> <a href="access.html" title="cpp/language/access">member access specifier</a> to derive from a base, all public and protected members of the base class are accessible as protected members of the derived class (private members of the base are never accessible unless friended)
</p><p>Protected inheritance may be used for "controlled polymorphism": within the members of Derived, as well as within the members of all further-derived classes, the derived class IS-A base: references and pointers to Derived may be used where references and pointers to Base are expected.
</p>
<h3><span class="mw-headline" id="Private_inheritance">Private inheritance</span></h3>
<p>When a class uses <code>private</code> <a href="access.html" title="cpp/language/access">member access specifier</a> to derive from a base, all public and protected members of the base class are accessible as private members of the derived class (private members of the base are never accessible unless friended).
</p><p>Private inheritance is commonly used in policy-based design, since policies are usually empty classes, and using them as bases both enables static polymorphism and leverages <a href="ebo.html" title="cpp/language/ebo">empty-base optimization</a>
</p><p>Private inheritance can also be used to implement the composition relationship (the base class subobject is an implementation detail of the derived class object). Using a member offers better encapsulation and is generally preferred unless the derived class requires access to protected members (including constructors) of the base, needs to override a virtual member of the base, needs the base to be constructed before and destructed after some other base subobject, needs to share a virtual base or needs to control the construction of a virtual base. Use of members to implement composition is also not applicable in the case of multiple inheritance from a <a href="parameter_pack.html" title="cpp/language/parameter pack">parameter pack</a> or when the identities of the base classes are determined at compile time through template metaprogramming.
</p><p>Similar to protected inheritance, private inheritance may also be used for controlled polymorphism: within the members of the derived (but not within further-derived classes), derived IS-A base.
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">template</span><span class="sy1">&lt;</span><span class="kw1">typename</span> Transport<span class="sy1">&gt;</span>
<span class="kw1">class</span> service <span class="sy4">:</span> <span class="kw1">private</span> Transport <span class="co1">// private inheritance from the Transport policy</span>
<span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
    <span class="kw4">void</span> transmit<span class="br0">(</span><span class="br0">)</span>
    <span class="br0">{</span>
        this<span class="sy2">-</span><span class="sy1">&gt;</span>send<span class="br0">(</span>...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// send using whatever transport was supplied</span>
    <span class="br0">}</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// TCP transport policy</span>
<span class="kw1">class</span> tcp
<span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
    <span class="kw4">void</span> send<span class="br0">(</span>...<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
<span class="co1">// UDP transport policy</span>
<span class="kw1">class</span> udp
<span class="br0">{</span>
<span class="kw1">public</span><span class="sy4">:</span>
    <span class="kw4">void</span> send<span class="br0">(</span>...<span class="br0">)</span><span class="sy4">;</span>
<span class="br0">}</span><span class="sy4">;</span>
 
service<span class="sy1">&lt;</span>tcp<span class="sy1">&gt;</span> service<span class="br0">(</span>host, port<span class="br0">)</span><span class="sy4">;</span> 
service.<span class="me1">transmit</span><span class="br0">(</span>...<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// send over TCP</span></pre></div></div>
<h3><span class="mw-headline" id="Member_name_lookup">Member name lookup</span></h3>
<p>Unqualified and qualified name lookup rules for class members are detailed in <a href="lookup.html" title="cpp/language/lookup">name lookup</a>.
</p>
<h3><span class="mw-headline" id="Defect_reports">Defect reports</span></h3>
<p>The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
</p>
<table class="dsctable" style="font-size:0.8em">
<tr>
<th> DR
</th>
<th> Applied to
</th>
<th> Behavior as published
</th>
<th> Correct behavior
</th></tr>
<tr>
<td> <a rel="nofollow" class="external text" href="https://cplusplus.github.io/CWG/issues/1710.html">CWG 1710</a>
</td>
<td> C++98
</td>
<td> the syntax of <span class="t-spar">class-or-decltype</span> made it impossible to derive from<br>a dependent class where the <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span></span></span> disambiguator is required
</td>
<td> allowed <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span></span></span>
</td></tr></table>
<h3><span class="mw-headline" id="See_also">See also</span></h3>
<ul><li> <a href="virtual.html" title="cpp/language/virtual">virtual functions</a>
</li><li> <a href="abstract_class.html" title="cpp/language/abstract class">abstract classes</a>
</li></ul>

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